This invention relates to growing crystalline bodies from a melt by the Edge-defined Film-fed Growth (xe2x80x9cEFGxe2x80x9d) process, and more particularly to improvements in apparatus for growing hollow crystalline bodies by the EFG process.
The EFG process is well known, as evidenced by the following U.S. Pat. Nos. 4,230,674; 4,661,324; 4,647,437; 4,968,380; 5,037,622; 5,098,229; 5,106,763; 5,156,978; and 5,558,712. In the EFG process crystalline bodies having a predetermined cross-sectional shape are grown on a seed from a liquid film of a selected feed material which is transported by capillary action from a melt contained in a crucible through one or more capillaries in an EFG die to the top end surface of the die. The shape of the crystalline body is determined by the external or edge configuration of the top end surface of the die. A major use of the EFG process is to grow polygonally-shaped hollow bodies of silicon, e.g., xe2x80x9cnonagonsxe2x80x9d or xe2x80x9coctagonsxe2x80x9d. These shaped hollow bodies are subdivided at their corners into a plurality of flat substrates that are used to form photovoltaic solar cells.
The preferred form of apparatus for growing hollow bodies by the EFG process comprises a capillary die/crucible assembly having a center hub which provides a passageway through which silicon particles are introduced to replenish the melt in the surrounding crucible during the growth process. In growing silicon bodies, the silicon particles are typically in the form of substantially spherical pellets having as size in the order of 2 mm. The particles are injected through he center hub into the space above the crucible, where they are deflected back down into the crucible. The common practice is to deliver the particles in predetermined quantities on an intermittent basis according to the rate of consumption of the melt, so as to maintain the level of the melt in the crucible within predetermined limits.
Growth of large thin-walled hollow bodies by EFG (e.g., silicon octagons in which each side or facet is 10 cm. wide) necessitates precise control of the heat input, since it is essential to maintain the temperature at the growth interface, i.e., in the meniscus region between the top end face of the die and the seed or the body grown on the seed, substantially constant at a level that allows growth to occur at a selected rate. In the EFG apparatus commonly used to grow hollow silicon bodies, heating is provided by induction heating coils that surround the furnace enclosure in which the crucible/die assembly is mounted. Thermal control of the growing crystalline body is achieved by controlling the heating power and also, inter alia, by use of concentric inner and outer after heaters between which the growing body is pulled away from the die. The afterheaters are, in effect, susceptors and are heated by electromagnetic induction. The inner and outer afterheaters help to control the thermal gradient lengthwise of the growing crystal and also affect the thermal gradient of the die and crucible in a radial direction, i.e., normal to the pulling axis.
Successful growth using the EFG process is complicated by the fact that variations in temperature tend to exist around the circumference of an EFG die and also radially of the die and crucible. Variations in thermal symmetry around the circumference of the die can cause local changes in thickness of the growing crystalline body. Such variations also make it difficult to sustain growth, often resulting in rupturing of the liquid menisci that extend between the die and the growing crystal body. When the menisci are ruptured, the growth stops.
Improvements in die design have reduced variations in thermal symmetry around the circumference of the die, thereby improving the quality of the grown bodies and reducing the rate of occurrence of rupturing of the menisci. However, even with improved die designs, EFG crystal growth apparatus of the type using crucibles with a center hub have been handicapped by a tendency for solid silicon to become attached to or grow on the center hub region of the crucible during crystal growth in response to disturbances in the growth zone. In this connection it should be noted that the thermal gradient in a radial direction is such that center hub of the crucible tends to be colder than the outer perimeter of the crucible.
It has been determined that prior EFG crystal growth apparatus lacks adequate means for controlling the path and speed of the particles as they travel out of the center hub and into the melt in the crucible, with the result that (a) sometimes the particles falling into the crucible cause splashing of the melt, with the result that liquid silicon impinges upon the upper portion of the center hub and (b) sometimes some of the solid particles come into direct contact with the upper end of the center hub. When this occurs, depending on the temperature of the center hub, the liquid silicon will solidify on and the silicon particles will become attached to and grow outward from the center hub, ultimately forming a mushroom-shaped solid mass that may be large enough to impede replenishment of the melt. Such solidification near the center hub region also affects the uniformity of the growing crystalline body and disrupts growth. Also fluctuations in temperature can result in pieces of the mushroom-shaped piece breaking off and failing into the melt, causing the crucible to overfill and flood the die.
Prior to this invention a common heater arrangement has comprised coaxial primary and secondary induction heating coils connected in series with a suitable, medium-frequency, power supply, with the primary coil having three turns and the secondary coil having a single turn and located above and spaced from the primary coil. The heater arrangement has also included a saturable reactor connected in parallel with the primary coil for the purpose of controlling the ratio of the currents flowing through the two coils. The saturable reactor allows the ratio of the currents to be adjusted, thereby modifying the temperature distribution along the axis of the furnace. However, saturable reactors suffer from the fact that they are costly, noisy and electrically inefficient.
A primary object of this invention is to provide an improved feed distributor/EFG crucible/die unit arrangement that controls the path of silicon particles as they move under gravity from a central feed tube to the crucible.
Another primary object is to provide an improved means for controlling the electromagnetic energy field used to heat an EFG crystal growth furnace.
A further object of this invention is to provide in an EFG crystal growth apparatus a particle feed distributor of novel design to control delivery of silicon particles into the melt-containing crucible so as to effectively eliminate or substantially reduce the occurrence of crystal growth on the center hub of the crucible.
Another object of the present invention is to provide in an EFG crystal growth apparatus for growing hollow bodies a combination particle distributor/inner afterheater assembly that substantially prevents or minimizes undesired solidification of melt material near the center region of the EFG die.
Still another object is to provide an improved method of delivering silicon particles into a crucible in an EFG crystal growth apparatus.
A further object is to provide an improved method of controlling the flow of electrical current in a pair of induction heating coils used to heat an EFG crystal growth apparatus.
Another object of the invention is to avoid solidification of silicon onto members associated with the EFG die, whereby to avoid premature termination of a growth run.
Still another object of the invention is to provide an improved method of growing a tubular crystalline body from a pool of melt.
The foregoing and other objects of this invention are achieved by modifying the particle distributor/inner afterheater structure associated with an EFG crucible/capillary die assembly, as typified by the apparatus disclosed in U.S. Pat. Nos. 4,661,324, 4,968,380, 5,037,622; and 5,098,229, so as provide particle distribution and flow control means that reduce the likelihood of silicon particles and splashed molten silicon contacting and adhering to relatively cold portions of the center hub section of the crucible and serving as the nucleus or site for further accretions caused by solidification of molten silicon from the melt, whereby to avoid interruption of the growth process and/or production of tubular bodies of poor quality. Control of the rate of heating is improved by using a Faraday ring to adjust the ratio of power supplied by the primary and secondary induction heating coils.
Other objects and features of the invention are set forth or described in the following detailed specification which is to be considered together with the accompanying drawings.